The present invention is directed to a ceramic discharge vessel for a high intensity discharge lamp and more particularly to the structure of parts of the discharge vessel that are in a green state during assembly of the discharge vessel.
Commercial ceramic discharge vessels typically include a polycrystalline alumina (PCA) ceramic. Alumina powder is mixed with a binder and the mixture is formed into a desired shape by isostatic pressing, extrusion or injection molding. The binders help a molded alumina part retain its shape while the part is in its “green state” (prior to binder removal and sintering).
U.S. Pat. No. 6,620,272, which is incorporated by reference, describes a method for assembling a ceramic body in which two ceramic halves of a discharge vessel are joined together. The surfaces to be joined are heated to cause localized melting and then brought together and joined at a seam by alternately compressing and stretching the seam. The joining apparatus described in this patent is shown in FIG. 1. The discharge vessel parts 1 are shown secured in the joining apparatus through the use of opposed holders 15. The discharge vessel parts and their corresponding holders are oriented to share common axis 12. Retractable pins 35 engage the electrode-receiving members (capillaries) 3 of the discharge vessel parts to hold the parts in place during joining. Once secured in the apparatus, the relative positions of the two parts are registered so that they may be accurately mated. Heater 19 is then interposed between the two holders and adjacent to the joining surfaces 7 of the discharge vessel parts. After heating, the parts are immediately brought together by displacing one or both of the holders toward each other along common axis 12. Compression is then applied by continuing to displace the parts in a forward direction toward each other and past the initial point of contact. The compression causes the softened material to bulge outward forming a visible seam. As the parts are brought together and compressed, the melted surfaces weld together to form a unitary body. At a predetermined point, the forward displacement of the discharge vessel parts is reversed and the parts are pulled away from each other causing a stretching of the material in the interface region. The stretching causes the still pliable material to thin thereby reducing the prominence of the seam. During stretching, the reverse displacement is preferably continued back through and beyond the initial point of contact between the sections.
This device is suitable for assembly of a discharge vessel from two green-state discharge vessel parts that have capillaries (3 in FIG. 1) on opposite sides that can be grasped by the device (e.g., pins 35) to facilitate the compression and stretching that reduce the prominence of the seam.
However, some discharge vessels parts do not have capillaries on opposite sides of a body and thus the device in FIG. 1 would not be suitable for assembling such discharge vessels. Discharge vessels with two capillaries on the same side of the body are disclosed in EP 1 111 654.
Further, regardless of whether the green-state discharge vessel parts are subject to the compression and stretching of the above-noted U.S. patent, green-state discharge vessel parts that are to be joined together must still be held in a stable and repeatable position to align the discharge vessel parts. The parts may pivot in their respective holders, causing misalignment of the capillaries, which must have a particular location and angle relative to each other in an assembled discharge vessel.